go/types/typeutil: make Hasher stateless

This CL makes Hasher an empty struct: consequently,
Hashers are stateless and all are equivalent.

Whatever optimization benefit Hasher had when it
was introduced is no longer evident: the benchmark
added in this CL went from 5.2ms (using the old
hasher) to 3.3ms (without). It also simplifies the
API and relaxes the concurrency constraints.

Fixes golang/go#69407

Change-Id: Ic6fa54451bee20cba72cd7133cf9afd38e7c3ca8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/612496
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

go/ssa/interp/value.go

@@ -99,10 +99,7 @@ var (
 // hashType returns a hash for t such that
 // types.Identical(x, y) => hashType(x) == hashType(y).
 func hashType(t types.Type) int {
-	mu.Lock()
-	h := int(hasher.Hash(t))
-	mu.Unlock()
-	return h
+	return int(hasher.Hash(t))
 }
 
 // usesBuiltinMap returns true if the built-in hash function and